what kind of foam is getting installed?
the difference in R value would be the thickness of foam applied.
make sure it is closed cell foam. open cell acts like a sponge.
One vendor's foam I've been using claims to be R7 per inch of thickness.

The operative will spray by hand, he will guess how much foam he is depositing as he goes. If they are trying to make money, he will probably not spray it on to the correct depth, working on the assumption that no one will ever know.
Get a specification in writing, confirming exactly what you are getting for your money, stand there and watch him do it.

Note: spray foam will shrink by 15% over time!
About 15 years, confirm that the specification is for then, or now!
Where it does shrink back, you will have a cold patch/patches, which will be subject to condensation/damp and possibly mold in time.